1. Field of the invention
This invention relates to cellular telephones, specifically to a method of multicast transmission for a direct sequence code division multiple access cellular telephone system.
2. Prior Art
Cellular telephone networks are commonly used for wireless two-way communication between a mobile station and a base station. Many base stations are interconnected to form a network. The base stations are connected to the land-line telephone network so the mobile stations may communicate with land-line telephones or other cellular telephones.
In a direct sequence code division multiple access (DS-CDMA) cellular system the communication between a mobile station (MS) and a base station (BS) is digital information. An example of a DS-CDMA cellular system is described in the TIA/EIA/IS-95A standard. In an IS-95A type system, the digital information may be data or compressed digitized voice. A BS normally will communicate over the forward link with more than one MS at a time by spreading the digital information with a unique spreading code assigned to each MS communicating on a common frequency. The digital information is also scrambled with a unique long code prior to spreading. A particular MS will only receive the information spread with its assigned spreading code and ignore the information transmitted to another MS. The information spread with other spreading codes appears as background noise and reduces the signal to noise ratio. Although a MS can receive information spread on any code channel it will be unable to descramble the information without knowing the long code used to scramble the data.
FIG. 1 is a block diagram of the signal processing in the forward link of a DS-CDMA receiver according to IS-95A. Long code generator 100 generates a pseudo random code used by adder 110 to scramble data. The long code is a serial bit pattern that is sequenced at a rate equal to the data rate being scrambled so there is no spectrum spreading. The long code is the modulo-2 inner product of a long code mask and the state vector of a sequence generator. A 64 bit Walsh code sequence is generated at the transmitted chip rate by Walsh code generator 120 and applied to adder 130 which achieves spectrum spreading.
Several frequency channels are used in each cell. Each forward link frequency channel uses a maximum of 64 spreading codes selected from a set of 64 orthogonal Walsh codes. The spreading code, scrambling code, and frequency channel are assigned to each MS as part of the service negotiation that occurs when a call is originated by the MS or is received by the MS.
Transmission of the same data or voice information to multiple MS is not directly supported by the IS-95A specification. It can be done by spreading the information with a different spreading code assigned to each MS intended to receive the transmission. Accordingly, the transmission to each MS requires one of the available 64 spreading codes. With each additional signal that is transmitted available system capacity is used as the signal to noise ratio is reduced.
It is impractical to transmit data or voice information to a large number of users because of the impact on system capacity. System capacity is limited by a number of factors including signal to noise ratio and the number of available spreading codes. Multicast transmission according to the prior art has the disadvantage of consuming an excessive amount of system capacity and being limited by the number of available spreading codes.
IS-95A type DS-CDMA uses frequency duplexing to separate the forward and reverse link. Communication from the BS to MS occurs on a forward link at one frequency, and communication from MS to BS occurs on a reverse link at another frequency. Simultaneous transmission occurs on both links. Additional signal traffic on either link increases the noise of that link, thus requiring higher transmission power to achieve an acceptable signal-to-noise ratio. When no information is transmitted on the reverse link, a low rate signal is still transmitted by the MS to maintain the synchronization of the BS receiver. This adds traffic and therefore reduces available system capacity. Capacity of the reverse link is generally the limiting factor in overall system capacity, so it is important to avoid unnecessary signal transmission on the reverse link.
If information is to be multicast to several mobile stations, a forward link is needed for the information. A reverse link is not normally needed because a multicast is a receive-only signal. According to the prior art a reverse link is still used, even though not needed. This causes the MS receiving the multicast to transmit a signal that is unnecessary and wasteful of battery power. The added signals on the reverse link also adds noise and requires other MS which are in two-way communication on that frequency channel to transmit at a higher power level. Multicast according to the prior art thus has the disadvantage of causing unnecessary signal transmission, wasting system capacity, and consuming excess battery power in the mobile stations receiving the multicast as well as mobile stations not receiving the multicast.
Conventional implementation of CDMA cellular communication as specified in IS-95A does not allow for efficient multicast transmission. An improved method of multicast transmitting is needed to efficiently transmit the same information to many mobile stations without limiting available system capacity and wasting battery power.